1. Field of the Invention
The invention concerns the field of reaction injection molded polyurethanes.
2. DESCRIPTION OF THE PRIOR ART
Reaction Injection Molding (RIM) is a technique for the rapid mixing and molding of large, fast curing urethane parts. RIM polyurethane parts are used in a variety of exterior body applications on automobiles where their light weight contributes to energy conservation. RIM parts are generally made by rapidly mixing active hydrogen containing materials with polyisocyanate and placing the mixture into a mold where reaction proceeds. These active hydrogen containing materials comprise a high molecular weight polyhydric polyether and a low molecular weight active hydrogen containing compound. After reaction and demolding, the parts may be subjected to an additional curing step which comprises placing the parts in an ambient temperature of about 250.degree. F. or greater.
Usual practice is to place all components except the isocyanate in one vessel and the isocyanate in another vessel (called the A component) prior to reaction and then mixing the A and B components together in the desired stoichiometric balance in a mold as discussed above. H. Peebles, Jr., Macromolecules 9 (1), pp. 58-61 discusses hard segment molecular weight distribution of a one stage thermoplastic polyurethane synthesis compared with a two stage synthesis. However, RIM is not discussed.
It has been surprisingly discovered that significant advantages occur when a significant portion or all of the high molecular weight polyhydric polyether is reacted with a portion of the isocyanate before the low molecular weight active hydrogen containing compound and the remaining high MW polyhydric polyether is mixed with the remaining unreacted isocyanate and the prereacted polyether and isocyanate and allowed to react.